Google's community manager, Michelle, in the AdWords forum said there may be "some edge cases" where switching your bidding strategy from any of the available options to maximize conversions bidding may result in high swings and spikes in your CPC, cost per click, prices.

We've been reviewing some edge cases where this happens - I'm sorry to hear you've been affected. We're working on a solution as I type. For now, you could switch to Target CPA or Target ROAS to apply an upper bound that you're comfortable with.

Here are some screen shots from advertisers showing the huge increase in cost when they make the switch:

It is unclear how many are impacted, but Michelle makes it sound like it is a really small number, as in an "edge case" that only impacts very few.

There are at least two threads with complaints about this in the Google AdWordsHelp Forums. So it isn't a huge explosion of complaints but a confirmed issue by Google.